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Radish seed germination (Raphanus sativus), coloured scanning electron micrograph (SEM). Root with close-up of root hairs. Seed germination is the growth of a plant contained within a seed. Seed germination depends on both internal and external conditions. The most important external factors include: right temperature, water, oxygen and sometimes light or darkness. The part of the plant that first emerges from the seed is called the embryonic root (also called the radicle or primary root). Root hairs are produced on the primary root in order to start water absorption. After the root absorbs water an embryonic shoot emerges from the seed. This shoot comprises three main parts: the cotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs among plant groups. Magnification: x75 when shortest axis printed at 25 millimetres.